As a working medium to be used for a heat pump system such as an air-conditioning system, a chlorofluorocarbon (CFC) such as chlorotrifluoromethane or dichlorodifluoromethane, or a hydrochlorofluorocarbon (HCFC) such as chlorodifluoromethane has been known. However, influences of CFCs and HCFCs over the ozone layer in the stratosphere have been pointed out, and their use is regulated at present.
As a working medium which has less influence over the ozone layer, a hydrofluorocarbon (HFC) such as difluoromethane, tetrafluoroethane or pentafluoroethane has been known. However, it has been pointed out that HFCs may cause global warming.
As a working medium which has less influence over the ozone layer and has less influence over global warming, a hydrofluoroolefin (HFO) having a carbon-carbon double bond which is likely to be decomposed by OH radicals in the air may be mentioned.
As a HFO with which excellent heat pump performance will be obtained, 1,1,2-trifluoroethylene (hereinafter referred to as “HFO-1123”) may be mentioned. However, HFO-1123 is flammable. To suppress the flammability of HFO-1123, a working medium having a HFC such as pentafluoroethane (hereinafter referred to as “HFC-125”) added to HFO-1123 has been proposed (Patent Document 1).